Indoleamine-accumulating amacrine cells are presynaptic to rod bipolar cells through GABA(c) receptors

gamma-Aminobutyric acid (GABA), is a main source of inhibitory modulation o f the rod pathway in the mammalian retina. The authors previously showed th at rod bipolar cells express at least three types of ionotropic GABA recept ors. Here, the authors sought to determine which neurons are the presynapti c partners at these synapses in the rabbit retina. Indoleamine-accumulating amacrine cells (IACs) were immunolabeled with an antiserum against seroton in (5HT) in vertical sections and wholemounts of rabbit retinae that had be en preloaded with 5HT The tissue was double labeled for the rho subunits of the GABA(C) receptor or the alpha 3 subunit of the GABA(A) receptor. Punct ate immunofluorescence was observed for both receptor subunits and was foun d to coincide with the dendrites and varicosities of IACs. The localization of rho subunits was examined at the ultrastructural level by using postemb edding techniques on slam-frozen, cryosubstituted tissue. Double labeling a t the electron microscopic level revealed that 5HT-immunoreactive processes were presynaptic to rod bipolar cells through GABA(C) receptors. Intracell ular injection of the two morphologic subclasses of IAC amacrine cells, S1 and S2, with Lucifer yellow followed by immunolabeling for the alpha 3 or r ho subunits revealed that varicosities on the dendrites of both cell types were in register with alpha 3- and rho-immunoreactive puncta. Taken togethe r, these results suggest that IACs are presynaptic to rod bipolar cells thr ough GABA(C) receptors and possibly through GABA(A) receptors. J. Comp. Neu rol. 413:155-167, 1999. (C) 1999 Wiley-Liss, Inc.